As per our scriptures there are 72000 nādis (subtle astral nerves) in a human body. They are those invisible nerves that transmit life-energy (prana shakti) throughout our body. The centre of the forehead is the focal point for all these subtle astral nerves. There is a possibility for the life-energy to flow out of our body from this point. In such case our life-energy gets depleted!
Kumkum, Vibhuti, sandal paste, Ganga mrittika, gopi chandan are the few items which have the capacity to prevent the life-force from draining out of this point. Hence sealing the centre of the forehead (bhru madhya) is the real idea behind this tradition of applying these items on the forehead! If worn neatly in a proper shape, it also protects the person from evil eye (drishti dosha). The bindi can be horizontal or vertical as per the individual’s customs.
Based on this logic, wearing Kumkum or Vibhuti on the forehead is equally important for men as it is for women!
These days, ladies wear stickers on the forehead and are contented that they are following the customs. However sticker does not come under the category of the above mentioned items and hence it is not useful from this angle.
What is the case of a widow who should not wear kumkum? Does she not have to protect her life-energy? Yes, she does. In such case, remember that the society requires her to not wear red-colour kumkum, which symbolizes married life, but does not stop her from wearing Vibhuti or any other item from that list mentioned above.